1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory device. More particularly, this is invention relates to a nonvolatile semiconductor memory device with MOS transistors each having a floating gate and a control gate.
2. Description of the Related Art
Nonvolatile semiconductor memories including NOR flash memories and NAND flash memories are widely used.
In recent years, a flash memory combining the features of the NOR flash memory and the NAND flash memory has been proposed. This type of flash memory has been disclosed in, for example, Wei-Hua Liu, “A 2-Transistor Source-select (2TS) Flash EEPROM for 1.8-V-Only Application,” Non-Volatile Semiconductor Memory Workshop 4.1, 1997. A flash memory of this type has memory cells, each including two MOS transistors. In such a memory cell, one MOS transistor functioning as a nonvolatile memory section has a structure including a control gate and a floating gate and is connected to a bit line. The other MOS transistor, which is connected to a source line, is used to select a memory cell.
Jpn. Pat. Appln. KOKAI Publication No. 2000-90680 and U.S. Pat. No. 6,222,774 have disclosed the following technique, used in the NOR flash memory. Specifically, they have disclosed the technique for preventing an adverse effect caused by the coupling between the word lines and the semiconductor substrate when the potentials of the word lines and semiconductor substrate are returned to 0 V after writing is done.
Unlike the NOR flash memory, the conventional flash memory with memory cells, each having two MOS transistors, has select gate lines. Therefore, not only the coupling between the word lines and the semiconductor substrate but also the coupling between the select gate lines and the semiconductor substrate exert an adverse effect on the operation. Particularly, taking into account the withstand voltage of the gate insulating film of the selected transistor, the methods disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-90680 and U.S. Pat. No. 6,222,774 are insufficient from the viewpoint of operation reliability.